Smokers who successfully stop smoking often resume the behavior after a brief period of time. A recently described biobehavioral model of smoking may provide insight into the complex problem of relapse among ex-smokers. The purpose of this proposed study is to examine the relationships of nicotine, b-endorphin, norepinephrine, and dysphoric states while smoking ad libitum, as well as during short-term deprivation and while on nicotine replacement therapy, to determine their role in contributing to a smoking behavior. A modified A-B-A-B experimental design will be used. Normal volunteers randomly assigned to be experimental group, will smoke ad lib, followed by an abrupt period of abstinence. Experimental subjects will then resume smoking ad lib, again followed by abstinence. During this second trial of abstinence, experimental subjects will be assigned to chew 2mq. nicotine gum or placebo gum. Dependent measures of plasma nicotine, plasma beta-endorphin, plasma norepinephrine and subjective reports of dysphoric states will be collected while subjects smoke ad lib, abstain from smoking, and abstain while on nicotine replacement therapy. This clinical Investigator Award will be conducted in a General Clinical Research Center at The Ohio State University. The sponsor of the project, Gopi Tejwani, PH.D., will provide additional research training for the Principal Investigator in his laboratory. Training will include learning an assay to measure beta-endorphins in human plasma.